package: avoid copying .m4 files from host to staging

Since we are preparing a package generation mechanism, we would like
to avoid packages that move/touch/modify files which are not part
of their own package. That's why we try to not install host files into
the staging directory (in this case .m4 files) and instead we include
$(HOST_DIR)/usr/share/aclocal into autoreconf search directory.

Signed-off-by: Lionel Landwerlin <llandwerlin@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>